1. Field of the Invention
The present invention relates to microwave processing and, more specifically, to a method of and apparatus for phosphor synthesis.
2. Description of Related Art
Microwave processing has been conducted on various substances (i.e., starting materials) including ceramics, composites, cermets, hard metals, electronic ceramics, metallic materials, etc. The general features of microwave processing of materials include volumetric and selective heating, enhanced reaction kinetics, the potential to improve product quality, process simplification, and the potential of cost reduction.
Microwave processing of ceramic materials is a dielectric heating process. The mechanism of microwave heating is inherently different from that of conventional heating. In microwave heating, heat is generated within the materials exposed to the microwave field through microwave-material interactions, whereas during conventional heating, heat is transferred from the heating element to the surface of the load by radiation and convection, then to the center of the load by thermal conduction. The absorption of microwave energy by the load in a microwave cavity depends on the dielectric loss factor of the materials in the microwave field. For the highly lossy materials, microwave processing can bring about substantial savings in time and energy with improved quality of the product. For example, regular, porous, and transparent hydroxapatite (HAp) ceramics have been fabricated by microwave processing within a few minutes; Ba(Zn1/3Ta2/3)O3 has been sintered to full density by microwave processing within 30 minutes at 1300–1400° C. compared to conventional sintering of the same material that requires 1600° C. and as long as 24 hours.
With reference to synthesis of fluorescent lamp phosphors, in order for activators to be incorporated into the crystal lattice structure of a host material, a high-temperature thermal treatment is necessary. Conventional processing of fluorescent lamp phosphors includes blending of the starting materials, loading the mixtures into crucibles, and firing at a high temperature for several hours. Additional finishing steps may include milling, washing to remove residual materials, filtering, drying, and blending. Although flux may be used to lower the firing temperature and accelerate the synthesis, the time length for synthesizing fluorescent lamp phosphors via conventional processing may still last anywhere from 8–12 hours. Further, contamination due to the volatiles from the conventional process can be of a concern. Additionally, the resultant phosphor obtained from a conventional process is a hard caked substance, thereby requiring the need to grind up the phosphor prior to utilization. As has been described, the conventional process of phosphor synthesis is not only complex, but also requires significant time and energy.
Therefore, it would be an advantage to lower the complexity and amount of time and energy utilized in obtaining fluorescent phosphors.